THERE is a correlation between the maximum blood glucose rise following the oral investion of lactose and jejunal lactase activity. Many patients with a low maximum blood glucose rise have normal lactase activity and so the test may only be employed as a screening test. Following the ingestion of 50 G lactose a maximum blood glucose rise of at least 1.5 mmol/l is required before one may assume normal lactase activity. A normal maximum blood glucose rise in the presence of low lactase activity is rare and is not associated with lactose intolerance.